Modern imaging systems using charged coupled device (CCD) arrays and complementary metal oxide semiconductor CMOS arrays have come into increasing use for both industrial and consumer applications. CCD arrays have a longer history and much more time to solve problems arising in manufacture and use. CCD arrays have fewer dark current problems than CMOS arrays. However, CMOS arrays promise less expensive imaging, principally because other functions may be combined on the semiconductor substrate and the well honed techniques developed for computer technology may be used to produce the arrays.
Prior art CMOS arrays have an array of pixels arranged in rows and columns, where light is absorbed in the semiconductor substrate to generate electrons, and the electrons are stored until they are “read out” to generate an image. The charge stored for one pixel is generally sent through a column amplifier to generate a voltage, which is then converted to a digital signal in an analog to digital converter (ADC). The ADC has a minimum signal input Vmin below which a zero is generated, and a maximum signal input Vmax which saturates the ADC. Each column amplifier generally has an offset voltage which is added to the signal from the stored electrons, so that the signal sent to the ADC is Vmin if no light is incident on the pixel and no electrons are stored. Since the amplification is slightly temperature and process dependent, the offset voltage must be adjustable, and means provided to send a “zero” or “dark” signal to the column amplifier so that the offset may be set to a voltage slightly less than Vmin. The “dark” signal may be sent by dummy pixels which in effect ground the input to the amplifier.
However, electrons are generated in the pixel without light by spontaneous means which are generally exponentially dependent on the temperature of the semiconductor material of the pixel. The electrons so generated give rise to “dark current”. Most light sensors produce “dark current”. The prior art of CMOS image arrays is deficient in that the “dark current” is not accounted for in the case that the array temperature, and also the variation in temperature over the array, changes with time. For the case of a CMOS integrated circuit application, where many functions are carried out on a monolithic semiconductor chip, this is a serious deficiency. Each part of the chip produces a different amount of heat at different times. Part of the array may be near a section of the chip having a dense array of devices liberating a large amount of heat, so that parts of the array will have a large temperature difference with respect to other parts of the array. The temperature and variation in temperature may change as the device is used for imaging, so that the dark current contribution to the image is variable and unaccounted for.